1. Technical Field
The present invention relates to a surface acoustic wave (SAW) element formed on a semiconductor substrate and a manufacturing method thereof.
2. Related Art
Ultrasonics Simposium 1989 Proceedings IEEE1989, 3-6 Oct. 1989, Pages 195-200 Vol. 1.: Visser, J. H; Vellekoop, M. J; Venema, A; van der Drift, E; rek, P. J. M.; Nederlof, A. J.; Nieuwenhuizen, M. S. is an example of related art. The example describes a surface acoustic wave (SAW) element and a manufacturing method thereof. To manufacture the SAW element, firstly, an epitaxial layer and a SiO2 layer are sequentially formed on a semiconductor substrate such as a Si substrate and the like. A semiconductor wiring region and a SAW region are arranged alongside each other in a plane on the SiO2 layer. In the SAW region, a comb-shaped inter digital transducer (IDT) made of aluminum and a piezoelectric layer (ZnO) are formed on the SiO2 layer which is thicker than the SiO2 layer in the semiconductor wiring region. In the semiconductor wiring region, an aluminum wiring layer and an insulating layer made of silicon nitride (Si3N4) are formed on the SiO2 layer.
According to the related art example, a height of the most upper layer in the semiconductor wiring region and a height of the SiO2 layer in the SAW region are different. More specifically, the most upper layer in the semiconductor wiring region is lower than the upper face of the SiO2 layer in the SAW region. Therefore, when the surface of the SiO2 layer in the SAW region is polished by chemical and mechanical polishing (CMP) and the like in order to make the surface flat and smooth for forming the IDT there, it was difficult to obtain a uniform thickness for the SiO2 layer in the SAW region where the IDT is formed. It is known that the speed (acoustic velocity) of the acoustic wave propagating the SiO2 layer in the SAW region differs depending on the thickness of the SiO2 layer where the IDT is formed. Since frequency is proportional to the acoustic velocity, if a region where the acoustic velocity is different from other regions exists in the element, this could lead to a problem that a stable frequency characteristic and a sharp band-pass property cannot be obtained.